The term blue generally describes colours perceived by humans observing light with a dominant wavelength that's between approximately 450 and 495 nanometres.

[4] In the United States and Europe, blue is the colour that both men and women are most likely to choose as their favourite, with at least one recent survey showing the same across several other countries, including China, Malaysia, and Indonesia.

[5][6] Past surveys in the US and Europe have found that blue is the colour most commonly associated with harmony, confidence, masculinity, knowledge, intelligence, calmness, distance, infinity, the imagination, cold, and sadness.

[11] The term blue generally describes colours perceived by humans observing light with a dominant wavelength between approximately 450 and 495 nanometres.

[16][17][18] In parallel, Isamu Akasaki and Hiroshi Amano of Nagoya University were working on a new development which revolutionized LED lighting.

[21] Nakamura, Hiroshi Amano and Isamu Akasaki were awarded the Nobel Prize in Physics in 2014 for the invention of an efficient blue LED.

[23] Previously the blue wavelengths were accessible only through DPSS which are comparatively expensive and inefficient, but still widely used by scientists for applications including optogenetics, Raman spectroscopy, and particle image velocimetry, due to their superior beam quality.

[24] Blue gas lasers are also still commonly used for holography, DNA sequencing, optical pumping, among other scientific and medical applications.

Prior to the 1700s, blue colourants for artwork were mainly based on lapis lazuli and the related mineral ultramarine.

A breakthrough occurred in 1709 when German druggist and pigment maker Johann Jacob Diesbach discovered Prussian blue.

The new blue arose from experiments involving heating dried blood with iron sulphides and was initially called Berliner Blau.

[27] In 1799 a French chemist, Louis Jacques Thénard, made a synthetic cobalt blue pigment which became immensely popular with painters.

In 1824 the Societé pour l'Encouragement d'Industrie in France offered a prize for the invention of an artificial ultramarine which could rival the natural colour made from lapis lazuli.

The prize was won in 1826 by a chemist named Jean Baptiste Guimet, but he refused to reveal the formula of his colour.

Cobalt is the blue chromophore in stained glass windows, such as those in Gothic cathedrals and in Chinese porcelain beginning in the Tang dynasty.

Natural ultramarine, made by grinding lapis lazuli into a fine powder, was the finest available blue pigment in the Middle Ages and the Renaissance.

[42] More commonly, blueness in animals is a structural colouration; an optical interference effect induced by organized nanometre-sized scales or fibres.

The appearance of blue, green, and hazel eyes results from the Tyndall scattering of light in the stroma, an optical effect similar to what accounts for the blueness of the sky.

Blue eyes are also found in parts of Western Asia, most notably in Afghanistan, Syria, Iraq, and Iran.

[57] Lapis lazuli artifacts, dated to 7570 BC, have been found at Bhirrana, which is the oldest site of Indus Valley civilisation.

Blue was not one of the four primary colours for Greek painting described by Pliny the Elder (red, yellow, black, and white).

[67] The Romans made extensive use of indigo and Egyptian blue pigment, as evidenced, in part, by frescos in Pompeii.

[69] By contrast, in the Islamic world, blue was of secondary to green, believed to be the favourite colour of the Prophet Mohammed.

At certain times in Moorish Spain and other parts of the Islamic world, blue was the colour worn by Christians and Jews, because only Muslims were allowed to wear white and green.

In the years that followed even more elegant blue stained glass windows were installed in other churches, including at Chartres Cathedral and Sainte-Chapelle in Paris.

[72] In the 12th century the Roman Catholic Church dictated that painters in Italy (and the rest of Europe consequently) to paint the Virgin Mary with blue, which became associated with holiness, humility and virtue.

The coat of arms of the kings of France became an azure or light blue shield, sprinkled with golden fleur-de-lis or lilies.

[73] Blue came into wider use beginning in the Renaissance, when artists began to paint the world with perspective, depth, shadows, and light from a single source.

[76] The early 19th century saw the ancestor of the modern blue business suit, created by Beau Brummel (1776–1840), who set fashion at the London Court.

Painter Mark Rothko observed that colour was "only an instrument;" his interest was "in expressing human emotions tragedy, ecstasy, doom, and so on".